Currently, as RAT that is Radio Access Technology, W-CDMA (Wideband-Code Division Multiple Access) specified in 3GPP (3rd Generation Partnership Project) is standardized as the 3rd generation cellular mobile communication system, and its services have been started sequentially (for example, see Non-patent Document 1).
Further, evolution has been studied in 3rd Generation RAT (Evolved Universal Terrestrial Radio Access, hereinafter referred to as “EUTRA”) and 3rd Generation RAT access network (Evolved Universal Terrestrial Radio Access Network, hereinafter referred to as “EUTRAN”). In EUTRA, as a communication system, an OFDMA (Orthogonal Frequency Division Multiplexing Access) system has been proposed (for example, see Non-patent Document 2).
In cellular mobile communication system, a mobile station, which is not assigned radio resources for a reason such as a state immediately after turn-on and the like, performs uplink transmission to a base station using the Random Access Channel (hereinafter, referred to as “RACH” as appropriate). Since the RACH is transmitted using radio resources that can be shared by mobile stations, it is considered that the RACH collides with another mobile station depending on the transmission timing. Therefore, mutually orthogonal data sequences are prepared, the orthogonal data sequence is transmitted on the RACH, and the base station is thereby capable of dividing received signals even with the same transmission timing when the signals are different data signal sequences. Such an orthogonal data sequence is called the signature, and a signal formed of the signature is called the RACH Preamble, and is used by a base station to identify a mobile station.
However, a collision occurs in the case that a plurality of mobile station selects the same RACH preamble at the same transmission timing, and in this case, retransmission processing is performed.
Uplink random access procedures in the W-CDMA system will be described briefly with reference to FIG. 17. FIG. 17 is a flowchart to explain transmission procedures of uplink random access channel in the W-CDMA system.
As shown in FIG. 17, a mobile station first calculates initial transmission power of the RACH preamble (step (hereafter, abbreviated as “ST”) 11). The initial transmission power is calculated in the mobile station from a measurement result of an intercell interference amount, downlink common pilot channel or the like. Next, the mobile station selects transmission timing of the RACH preamble (ST12), and subsequently, randomly selects one from among a plurality of signatures to generate the RACH preamble (ST13). Then, the mobile station transmits the RACH preamble with the initial transmission power at the transmission timing to a base station (ST14).
After transmitting the RACH preamble, the mobile station determines whether ACK (Acknowledge) indicative of transmission permission is returned from the base station (ST 15). Herein, when ACK is returned, the mobile station starts actual data transmission called the RACH message (ST 16). Meanwhile, when ACK is not returned from the base station, or NACK (Not Acknowledge) is returned, the mobile station checks whether or not the number of retransmissions beforehand defined expires (ST 17). When the number of retransmissions does not expire, the mobile station increases the transmission power (ST 18), newly selects transmission timing for retransmission, and generates the RACH preamble from a randomly selected signature to retransmit (ST 12, ST 13). In the case where the mobile station checks whether or not the number of retransmissions beforehand defined expires at ST 17, while repeating the same processing, and cannot receive ACK from the base retransmissions expires, the mobile station judges as RACH transmission failure (ST 19), and finishes a series of procedures.
In addition, due to differences in wireless system, EUTRA requires control different from the above-mentioned random access procedures in the W-CDMA system. FIG. 18 is a diagram showing an example of channel mapping of RACH proposed in EUTRA. In this example, the RACH uses a region of 1.25 MHz within system frequency bandwidth BW, and further, uses a one-subframe interval TTI (Transmission Timing Interval) in the time domain. In addition, since the RACH is used in a stage that uplink synchronization has not been acquired, guard time is required prior and subsequent to actual RACH transmission for the purpose of preventing interference of transmission data due to propagation delay.
In EUTRA, it is studied that the random access channel is used for the purposes of a mobile station registering the position, notifying handover to a base station that is a handover destination, requesting radio resources, transmitting data in intermittent transmission, maintaining uplink radio synchronization, and the like.
In EUTRA, due to the relationship of the TTI length of the RACH and transmission bandwidth, it is anticipated that the number of data bits that can be contained in the RACH is lower than that in the W-CDMA system. Therefore, a method is required for implicitly notifying information without using data that is actually transmitted. Non-patent Document 3 proposes a method of notifying information using a number of a signature that is a data sequence included in the RACH preamble. Herein, this method is explained with reference to FIG. 19.
FIG. 19 shows an example in which there are 32 data sequences usable as the RACH preamble, and numbers of signatures to use are classified according to a reason of RACH transmission and quality information indicator (Channel Quality Indicator (hereinafter, referred to as “CQI”)) at the transmission. For example, when the RACH is transmitted at initial transmission, and the CQI of the mobile station at the transmission is classified as “High”, the mobile station selects one from among signature numbers 3 to 5 as shown in FIG. 19, and transmits the RACH preamble. By using this method, without being included in actual transmission data, the base station is capable of grasping the reason of RACH transmission and CQI of the mobile station from the received signature number.
Further, in EUTRA, technique called interference coordination is proposed to reduce an uplink intercell interference amount (for example, see Non-patent Document 2). A plurality of methods is proposed as the interference coordination, and as a predominant method, such a method is proposed that the frequency domain usable in the system is divided into some regions, mobile stations are divided into a plurality of groups based on the transmission power, downlink reception quality (path-loss, or CQI) and the like, each group is associated with the divided frequency region, and that the mobile stations perform transmission only in the associated frequency regions (for example, see Non-patent Document 4). Non-patent Document 4 introduces a method of setting higher target quality of base-station received power in the associated frequency region as the mobile station comes closer to the base station, and thereby improving throughput of transmission data without increasing the uplink intercell interference amount. Herein, this method is explained with reference to FIGS. 20 and 21.
FIG. 20 is a diagram showing that mobile stations UE_A and UE_B are located in cells A to C. Herein, it is assumed that UE_A and UE_B communicate with the cell A. At this point, since UE_A is located near the center (base station) of the cell A, it is considered that downlink reception quality of UE_A is good, and that at the same time, the uplink intercell interference amount hardly exists to peripheral cells (cell B and cell C). Meanwhile, since UE_B is located in the cell edge of the cell A, it is considered that the downlink reception quality is poor, and that at the same time, the uplink intercell interference amount is large to the peripheral cells (cell B and cell C). Therefore, UE_B needs to set the target quality low to reduce the uplink intercell interference amount to the peripheral cells. Meanwhile, UE_A provides few uplink intercell interference amount to the peripheral cells, and is better to set the target quality high to improve uplink throughput.
FIG. 21 is a diagram showing an example for setting different target quality for each frequency region. In FIG. 21, the frequency range of the system is divided into four regions of from RU_BW1 to RU_BW4. Further, mobile stations in the cell are grouped to four stages according to the reception quality, and are assigned the regions starting with RU_BW1 for an excellent quality group. In other words, mobile stations belonging to the poorest quality group use RU_BW4. The target quality of mobile stations belonging to RU_BW4 is Target A, and the target quality is set higher by STEP_n that is a predetermined step width as the quality of the group increases. By this method, it is possible to improve uplink throughput without increasing the uplink intercell interference amount to peripheral cells.
In addition, as the target quality, assumed is SIR (Signal-to-Interference Ratio), SINR (Signal-to-Interference plus Noise Ration), SNR (Signal-to-Noise Ratio), path-loss or the like.
Non-patent Document 1: Keiji Tachikawa, “W-CDMA mobile communication system”, ISBN4-621-04894-5, Initial print on Jun. 25, 2001, Maruzen Co., Ltd
Non-patent Document 2: 3GPP TR(Technical Report) 25.814, V1.5.0 (2006-5), Physical Layer Aspects for Evolved UTRA. http://www.3gpp.org/ftp/Specs/html-info/25814.htm
Non-patent Document 3: NTT DoCoMo. et al, “Random Access Channel Structure for E-UTRA Uplink”, 3GPP TSG RAN WG1 Meeting #45, Shanghai, China, 8-12 May, 2006, R1-061184
Non-patent Document 4: Nokia, “Uplink inter cell interference mitigation and text proposal”, 3GPP TSG RAN WG1 Meeting #44, Denver, USA, 13-17 Feb., 2006, R1-060298